Isothermal crystallization behavior of high-density polyethylene (HDPE) was studied experimentally by using a rotary rheometer with parallel plates fixtures.It was found that the crystallization rate depended on the surface roughness of the fixtures with the same chemical composition.As the surface roughness increased,the crystallization rate first increased then decreased which meant that there was a range of surface roughness where the crystallization rate could reach a maximum value.Surface roughness effectively enhanced the interfacial area and decreased heat resistance, but possibly trapped air pockets in the valley of the rough surface produced a considerable interfacial heat resistance,both affecting the crystallization rate of HDPE.The isothermal crystallization rate decreased in sequence as the HDPE specimen contacted with aluminum,brass and stainless steel plates whose surface roughness were on the same level.The sample crystallizes more quickly on the fixtures with higher heat conduction coefficient, thus lowering heat resistance.
The experimental observations about remarkable influence of the substrates on the isothermal crystallization rate of a high density polyethylene(HDPE) were presented.Two methods were used to characterize the crystallization rate:the change of turbidity of the HDPE specimen and the changes of the complex viscosity and storage modulus measured by a rotational rheometer,which gave consistent results showing that the isothermal crystallization rate decreased in sequence as the specimen contacted with aluminum,brass and stainless steel plates,respectively.As to the dominant influence factor,the chemical composition of the substrates can be excluded via insulating the plate by an aluminum foil.Instead,we propose the plate's ability of removing the latent heat of crystallization from the specimen.Rheological measurement is sensitive to the crystallization process.The colloid like model proposed by BOUTAHAR et al for the crystallization of HDPE gives reasonable predictions of the crystallized fraction from the measured storage modulus.
